Karl Hampton, Rachel Bott, Seyi Latunde-Dada, Oksana Leszczyszyn
Malvern Instruments Ltd, Malvern, Worcestershire, UK
TAYLOR DISPERSION ANALYSIS: A TOOL FOR
MASS-WEIGHTED, UV SELECTIVE ANALYSIS OF
HYDRODYNAMIC SIZE
Introduction
Evolution of trace data
DISPERSION
Taylor Dispersion is a model that describes the behaviour of a sample plug as
it moves through a pipe under Poiseuille flow. Analysis of this model allows
the determination of the diffusion coefficient and hydrodynamic radius for
molecules within the sample plug.
The Viscosizer TD uses Taylor Dispersion theory in its Sizing measurement.
There are two important
transport mechanisms in Taylor
Dispersion: these are convection
and diffusion.
DIFFUSION
Signal
A plug flowing through a
capillary forms discrete layers
– laminae – which travel at
different velocities.
Laminae closer to the wall of
the capillary move slower than
central laminae.
t0
t1
t2
t3
Time
t4
t5
tn
Above: A schematic showing the evolution of a Taylorgram (trace data) in the Viscosizer TD (top
right) during a Sizing measurement.
Above: Transport mechanisms in Taylor Dispersion
These differential velocities cause the sample plug to spread out in the
axial direction. Consequently, a concentration difference is established
between the sample and buffer boundary, and allows diffusion in the radial
direction.
Size differentiation using Taylor Dispersion
LARGE MOLECULES
SMALL MOLECULES
The interplay of dispersion and diffusion influences the shape of the plug
as it moves through the capillary. The degree to which each of these
mechanisms act upon the plug is an inherent property of the molecular size
of species in the plug.
A small plug of several nanolitres is injected (t0) into a microcapillary. UV
detection is used to selectively monitor the absorbance at cross-sections of
this plug (t1 to tn). Absorbance is plotted as a function of time and produces a
concentration profile at each detection point.
Archetypal Taylorgrams
With two detection points, the action of dispersion and diffusion on the
sample plug as it moves along the capillary can be observed; whereby the
second peak of each Taylorgram is broader than the first. The impact of
molecular size on plug broadening is also evident between Taylorgrams.
Small molecule
Caffeine
Rh 0.33 nm
• Small molecules diffuse
quickly in the radial direction
• Diffusion works against
dispersion
• Keeps the plug compact and
peak narrow
Protein
IgG
Rh 5.58 nm
Two-component
Caffeine + IgG
Rh 5.47 nm
Rh 0.33 nm
• Diffusion occurs slowly in the
radial direction
• Plug dispersion is more
dominant
• Peak broadens
Above: Representative Taylorgrams of small molecules (A), proteins (B) and mixtures (C)
recorded by the Viscosizer TD. Rh obtained by Taylor Dispersion Analysis are also provided.
• The molecular diffusion
coefficient (D) is inversely
proportional to width of a
Gaussian peak
The detection of different size populations in mixtures is possible thanks to the
use of UV detection, which provides a mass-weighted analysis. The ability to
observe the contribution of a molecule using a property that is independent of
size offers potential advantages in the characterisation of challenging samples
in formulation development. Such samples include those generated during
stability studies, e.g. aggregated solutions, excipient-laden formulations or
those in complex biological media.
Above: Characteristic plug shapes and corresponding concentration
profiles of small and large molecules (top and middle panels).
The relationship between the molecular diffusion coefficient (D)and peak
width (t); where RC is the capillary radius and t0 the residence time.
© Malvern Instruments Limited 2015
Material relationships
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